1. Field of the invention
The present invention relates to a process for manufacturing a superconductor having a high critical temperature and a method for producing a superconducting circuit. More particularly, it relates to a method for producing a circuit pattern of a superconductor having a high critical temperature which is used in the field of electronics.
2. Description of the related art
The superconductivity is a phenomenon which is understood as a phenomenon of phase change of electrons under which the electrical resistance become zero and the perfect diamagnetism is observed. Thus, under the superconducting condition, electric current of a very high current density can be delivered without any loss of power. Therefore, the power loss in circuits used in the field of electronics can be reduced greatly. It is also expected to utilize the superconductor in the field of a high sensitive sensors or detectors for sensing very weak magnetic field, microwave, radiant ray or the like.
However, the critical temperature could not exceed 23.2K of Nb.sub.3 Ge which was the highest Tc heretofore for all studies for the past ten years.
Possibility of existence of a new type of superconducting materials having much higher Tc was revealed by Bednorz and Muller who discovered a new oxide type superconductor in 1986 [Z. Phys. B64 (1986) 189].
It had been known that certain ceramics material of compound oxides exhibit the property of superconductivity. For example, U.S. Pat. No. 3,932,315 discloses Ba-Pb-Bi type compound oxide which shows superconductivity and Japanese patent laid-open No. 60-173,885 discloses that Ba-Bi type compound oxides also show superconductivity. These type superconductors, however, possess a rather lower transition temperature of about 10K and hence usage of liquidized helium (boiling point of 4.2K) as cryogen is indispensable to realize superconductivity.
This new oxide type superconducting material discovered by Bednorz and Muller is [La, Ba].sub.2 CuO.sub.4 which is called as the K.sub.2 NiF.sub.4 -type oxide having a crystal structure which is similar to known perovskite type oxides. The K.sub.2 NiF.sub.4 -type oxides show such higher Tc as 30K which are extremely higher than the known superconducting materials.
It was also reported in the newspaper that C. W. Chu et al discovered in the United States of America another type of superconducting material having the critical temperature of in the order of 90K in February 1987, and hence possibility of existence of high-temperature superconductors have burst on the scene.
However, the above mentioned new type superconducting materials which was just born and which are presumed to be classified in the perovskite crystals of La-Ba-Cu-O system and Y-Ba-Cu-O system have been studied and developed mainly in a form of sintered articles. In already known methods, the superconducting materials are compacted in a metal pipe and then are drawn to produce a superconducting wire or they are blended with organic binder to prepare a paste for circuit printing. These method, however, include inevitably the final sintering stage of the superconducting material shaped into a wire-form or a layered form, so that higher dimensional stability of the sintered product can not be expected due to shrinkage which occur during the sintering operation and it is difficult to produce a variety of articles having desired configuration freely and precisely. In other words, a finely patterned circuit for electronics devices can not be produced by the conventional methods. Still more, the products do not possess satisfactory properties of superconductivity because the oxygen contents which is one of the critical factors of this type of superconductor can not be controlled or adjusted to an optimum value for realizing higher critical temperature.
It may be also used a variety of vapour phase deposition techniques to produce a thin film of superconductor which is applicable to electric circuits having fine patterns. In fact, in these techniques, it is possible to deposit the superconducting material on a substrate to produce a fine circuit pattern by means of a masking sheet. Such superconducting circuit pattern may be produced also by the ion-etching or ion-sputtering technique in which predetermined portions of the thin film of superconductor deposited on the substrate is cut or removed to leave a circuit pattern.
However, in the conventional sputtering technique for preparing a thin film of superconductor, sputtering must be carried out in the presence of oxygen gas added in a sputtering gas and also the substrate must be heated at such a temperature that an improved crystalline structure is obtained. Still more, the resulting sputtered film must be heat-treated in an atmosphere containing oxygen gas to increase the oxygen contents in the thin film. For example, in Japanese patent laid-open No. 56-109824 which discloses a method for preparing a thin film of superconductor represented by BaPb.sub.1-x Bi.sub.x O.sub.3 (in which 0.05.ltoreq.x.ltoreq.0.35), high-frequency sputtering is carried out in an oxygen containing atmosphere and then the resulting film must be further heated at 500.degree. to 550.degree. C. We also described sputtering conditions for preparing superconducting thin films by the sputtering technique in co-pending U.S. patent Ser. No. 152,714 filed on May 2, 1988.
The abovementioned conventional sputtering technique itself is satisfactory but is difficult to control the nature of gas in the atmosphere, resulting in that the crystal structure in a thin film obtained can not be controlled precisely.
Still more, it is not only difficult to perform the patterning operation of superconducting circuits easily and precisely on the thin film produced by the abovementioned sputtering technique but impossible to provide a superconducting circuit of high quality.
Therefore, the abovementioned conventional techniques are not suitable for production of patterned superconducting circuit on a substrate.
An object of the present invention is to overcome the abovementioned problems of the conventional technique and to provide a process for controlling the crystal structure more precisely, particularly for adjusting the oxygen contents in a superconductor.
Another object of the present invention is to provide a simple method for producing a fine superconducting circuit pattern having improved superconducting property on a substrate.